Distribution of Crataegus (Rosaceae) along a gradient of anthropogenic influence

The impact of land use intensity on the taxonomic composition of hawthorns in four regions of south-eastern Poland was investigated. As a result of different human settlement histories, two of the regions are 54–64% forested nowadays, and the remaining two regions are 70–78% transformed into arable land. Numerical analysis and Monte Carlo permutation tests showed that the purely geographical component of the species distribution, linked with the land use intensity, was statistically significant (1.97%, P = 0.002), as well the pure site component (1.17%, P = 0.010). The human-induced opening of the landscape promotes Crataegus monogyna. Forests are occupied by C. laevigata, C. rhipidiphylla, C. rhipidophylla Gand. var. lindmanii and the hybrid C. × macrocarpa (C. rhipidophylla × C. laevigata); on forest edges the hybrid C. × media (C. monogyna × C. laevigata) tends to occur. Crataegus × subsphaericea (C. monogyna × C. rhipidophylla) is rare and tends to occur in thickets, and a triple hybrid C. monogyna × C. rhipidophylla × C. laevigata colonizes recently abandoned fields. Hybridization seems to be an efficient evolutionary strategy of hawthorns in the face of human-induced transformations of the landscape.